I had this problem before I could even change classes. My solution was to not touch the storyline quests till I could change classes and start into crafting and gathering classes so I could put that exp somewhere. Let me tell you, though, that after a point the quests alone stop giving you the exp to get to the next level. I've been having to supplement with leves if I want to use that quest exp (who thought it a good idea to not let Land or Hand benefit from Fates?).